PURPOSE: To judge whether an ignition system is normal or not by detecting and averaging the level and the duration time of ignition in every cylinder for comparing these values with those of each cylinder.

CONSTITUTION: A data processing unit 42 calculates an ignition peak voltage VP and a discharge time TD of each cylinder to obtain each averaged VP and inversed TDand calculate vpi=VP and tdi=TDi/inversed TD. When vpi>lower limit reference value and tdi>upper limit reference value, it outputs a result that leakage is caused in an ignition plug and a distributor in a specific cylinder, and when vpi>upper limit reference value and tdi<lower limit reference value, it outputs a result that they are released. This construction permits to diagnose the good or the bad of various engine ignition systems correctly.